Silver halide photography involves the exposure of silver halide with light in order to form a latent image that is developed during photographic processing to form a visible image. Silver halide is intrinsically sensitive only to light in the blue region of the spectrum. Thus, when silver halide is to be exposed to other wavelengths of radiation, such as green or red light in a multicolor element or infrared radiation in an infrared-sensitive element, a spectral sensitizing dye is required. Sensitizing dyes are chromophoric compounds (usually cyanine dye compounds) that are adsorbed to the silver halide. They absorb light or radiation of a particular wavelength and transfer the energy to the silver halide to form the latent image, thus effectively rendering the silver halide sensitive to radiation of a wavelength other than in the blue region of intrinsic sensitivity.
The advent of solid state diodes that emit red and infrared radiation has expanded the useful applications of infrared-sensitive photographic elements. The diodes have a wide variety of emission wavelengths, ranging from around 660 nm to around 910 nm. Typical emission wavelengths include 750 nm, 780 nm, 810 nm, 820 nm, and 870 nm. Because of the wide variety of emission wavelengths, it would be desirable for an infrared-sensitive photographic material to have broad sensitivity in the infrared region of the spectrum. This would allow a single material to be used with a diodes having a variety of emission wavelengths.
Such broad sensitivity can generally be provided by either using a single sensitizing dye that provides broad sensitivity or by a combination of sensitizing dyes (usually two) that, by themselves, would provide narrower sensitivity. Many such broad sensitizing dyes can suffer from a number of problems, such as poor keePing stability (e.g., formation of fog during keeping) and poor safelight performance. Many dye combinations also have disadvantages, such as poor sensitivity (e.g., due to desensitization) or poor keeping stability (e.g., formation of fog during keeping).
It is an object of this invention to provide silver halide with broad sensitivity in the infrared region of the spectrum without incurring the above-described problems.